Overheating is a serious (and possibly fatal) problem for computers.
When a processor gets too hot the computer will turn its self off immediately which can cause you to loose unsaved work,
and may damage your hard drive.
If your computer is starting to suffer from overheating you should have it cleaned ASAP.
To reduce the amount of fluff that your computer's fan draws in, avoid putting it on beds, clothes or other fabrics.
Signs of overheating include:
Switching its self off while in use
Lots of fan noise
Laptops feeling especially hot underneath
Dust and fluff in a fan and vent blocking airflow.